(1) Field of the Invention
This invention relates to a mask, a method for forming a pattern, and a method for evaluating the line width of a pattern and, more particularly, to a mask used for evaluating the line width of a mask pattern formed on a photo mask, a method for forming such a pattern, and a method for evaluating the line width of such a pattern.
(2) Description of the Related Art
Control of the line width, such as the gate line width, of circuit patterns in semiconductor devices is very important in fabricating them. To form a circuit pattern, a photo mask where a mask pattern which is usually four or five times the size of the circuit pattern is formed is used. This mask pattern is reduced and transferred onto a wafer by exposure. Accordingly, when a photo mask is fabricated, the line width of a mask pattern formed on it must be accurate.
For example, with an ordinary binary photo mask, a light shielding pattern is formed on a silica substrate with a film of metal, such as chromium (Cr) or molybdenum silicide (MoSi). That is to say, light transmitting portions which transmit light at exposure time and light shielding portions which shield light at exposure time are formed.
In recent years circuit patterns in semiconductor devices have become minuter, so mask patterns formed on photo masks have also become minuter. In many cases, scanning electron microscopes (SEMs) are used for checking whether such minute patterns are formed on photo masks with great accuracy.
However, photo masks are insulating silica substrates on which a conductive metal film is formed. Therefore, when the line width of a mask pattern is determined with a SEM, the influence of secondary electrons emitted from the surface of the photo mask changes due to a change in the aperture rate or covering rate of the mask pattern in an area near a portion where determination is made, and a charge-up of insulating portions often occurs. As a result, a SEM image which is being observed may gradually shift, that is to say, what is called a flicker of a SEM image may occur or the mask pattern may look to be expanded. In many cases, this causes an error in the line width of the mask pattern determined on the basis of the SEM image.
Conventionally, various methods have been proposed to solve problems caused by charge-up which occurs at the time of using a SEM. For example, a method for using a sample on which insulating patterns with known line width are formed alternately with conductive patterns with known line width and for clarifying the limitations of determination of line width by a SEM according to the amount of a change in the line width of the insulating patterns in respect to the line width of the conductive patterns at the time of observation by the SEM is proposed (see, for example, Japanese Unexamined Patent Publication No. Hei 8-288352).